The present invention relates to the lumber industry, and particularly to cutting and/or shaping of lumber as part of the drying process and to minimize warpage.
Dimension lumber is defined in the US as lumber with a nominal thickness of from 2 inches up to 4 inches and a nominal width of 2 inches or more. Most of such lumber is of nominal 2 inch thickness. In the U.S., softwood dimension lumber in excess of 19% average moisture content (“MC”) is defined as “unseasoned”. Framing lumber of nominal 2 inch thickness must not exceed 19% MC to be grade stamped “S-DRY.” S-DRY lumber is generally more dimensionally stable and stronger than unseasoned or green lumber and therefore commands a higher price, and significant cost and equipment has been used to attempt to rapidly and efficiently dry lumber to the S-DRY grade.
One of the primary factors hindering rapid and quality drying of softwood dimension lumber is the inherent lack of permeability of the wood. It is well accepted that moisture moves within the board parallel to the grain of the wood markedly easier than perpendicular to the grain. Moisture moving a given distance parallel to the grain encounters only a fraction of the cell wall substance encountered over the same distance perpendicular to the grain. It is stated in the literature that moisture travels about 15 to 20 times faster through end grain than side grain. For example, in an 8 foot long 2×4 board, the two ends quickly dry for some distance along the grain. In the remainder of the board, drying must occur by transmission of moisture through the side grain, i.e. perpendicular to board length. In a green 8 foot nominal 2×4 board, there is less than 13 in2 of exposed end grain, but nearly 1100 in2 of exposed side grain. Consequently, in spite of fast drying through the end grain, most of the overall drying must occur through side grain.
Most drying of nominal 2 inch thick dimension lumber occurs in a kiln to an average of 14 to 15% MC prior to being “surfaced four sides” (S4S) and then grade stamped. The resulting range in MC for the thousands of boards in a single kiln run is about 4% to 19%, or often higher than 19%. The pieces in the 4% to 8% range are over dried and thus have warped excessively, principally in the forms of crook, bow, and twist. With strict limits on the allowable amount of warp for a given grade of the lumber, the warp degrade translates into an immediate loss in value. The severe warp also adversely affects the ability to S4S the lumber. Pieces of higher MC, in the range of 13% to 19% or higher, can undergo post drying during storage and transport or in the context of structural incorporation. The post drying and associated warp fuels further economic loss and depreciates overall customer acceptance of the product. Drying to a lower average MC and narrower range in MC, while minimizing warp, should produce both higher economic return and customer satisfaction.
In the drying of contemporary lumber, essentially all moisture movement must take place perpendicular to the grain. This causes steep MC gradients within the boards that result in severe drying stresses. The increased drying stresses typically result in increased warpage.
Most of the dimension lumber produced is utilized for framing in which loading is perpendicular to a narrow edge. For softwood dimension lumber used as floor joists, rafters, door headers, etc. the major strength requirement is bending strength for loading perpendicular to the narrow edge. The use of wider pieces, e.g. the nominal 10 and 12 inch widths for floor joists, headers etc., has decreased rather dramatically over the past 2 or more decades. One factor contributing to the decreased use of wide dimension lumber is the harvesting of smaller trees. A second and equally important reason is the unreliable dimensional stability of the currently produced solid lumber. Recent commentary states that nearly 90 percent of floors for new homes in California use engineered I-Joists rather than solid lumber and then goes on to say that in a survey of U.S. building contractors lack of “straightness” was what made them least satisfied with solid lumber.
Bending strength is understood to be highly dependent on the moment of inertia, commonly designated as “I”. For a rectangular cross section, the I value is determined as:I=bd3/12in which b=breadth and d=depth. For a seasoned, nominal S4S 2×12, the I value is:I=1.5 inches×(11.25 inches)3/12=178 inch4When used as a floor joist e.g. the stress in bending equals the bending moment times d/2 divided by the I value. The dominating effect of I value upon stress is quite apparent.
The cross section of a selected engineered wood I-joist has the following dimensions: depth=11 inches, top and bottom flanges each 2.5 inches wide by 1.4 inches deep, and the web member of 3 layer plywood is 0.35 inches thick with a clear span depth of 8.2 inches. Its numerical I value is 178 inches4. As shown above, the numerical I value for a seasoned nominal 2×12 is 178 inches4. The engineered I-joist thus appears designed to replace the 2×12, doing so with only 60% of the cross sectional area of the 2×12.
Improved drying both within and between individual lumber pieces has been long desired. Some pretreatments, such as presteaming or prefreezing, have proved beneficial for certain species. However, these are difficult and expensive for incorporation into the contemporary production lines common for construction lumber.